The invention concerns a method for quality control of superconducting bands, in particular high-temperature superconductor (HTS) bands, in respect of the critical current carrying capacity of the band. The invention further concerns a device for carrying out said method.
The critical transport current Ic is one of the most important parameters of high-temperature superconductor materials for industrial technical applications. In that respect the critical transport current specifies the maximum current at which an electric component is still superconducting, at greater transport current strengths superconduction collapses and the component becomes inoperable because of the electric resistance of the current conduction path, which is increased as a result, wherein the generation of heat that is linked thereto can even lead to destruction of the respective component. For the technical use of superconducting electrotechnical components or devices therefore it is necessary that they are superconducting at least at a critical transport current Ic (min) which is the minimum in dependence on the respective situation of use.
In the industrial production of superconducting bands which is therefore not effected under laboratory conditions, the respective band can suffer to a particular degree from defects which limit the critical transport current. Those defects can be of differing nature, for example in the production of superconducting bands by chemical reactive depositions of the materials due to local non-stoichiometries, locally incomplete reactions of the precursors used, irregularities in respect of thermal decomposition reactions or tempering of the respective intermediate or end products, local defects in the case of epitaxial growth of the superconductor material on the respective substrate and the like. Such defects can limit the critical transport current in such a way that the respective band is to be considered as scrap for the respective purpose of use or however can be used only for lower technical requirements. Quality control of superconducting bands is particularly relevant in that respect when those bands for commercial technical applications are of a great length, for example for the production of superconducting coils or electric supply lines, wherein those bands can readily be of a length of several hundreds of meters or several kilometers.
In addition the method according to the invention aims to be suitable for permitting quality control in the case of superconducting bands which are produced on a large technical scale, for technical applications, so that the method is suitable for permitting quality control in a production process for bands with a production capacity of an installation of several kilometers of band per day.
EP 860 705 B1 discloses a method and a device for measuring the critical current value of a superconductor wire. The superconductor wire (notionally) comprises a multiplicity of longitudinally arranged wire portions. A multiplicity of electric currents of differing current strength is predetermined, wherein a respective current of a respective given current strength is passed repetitively through the respective superconductingly cooled band portion and the voltage generated in the respective wire portion is detected until all current strengths and the resulting voltages for that band portion are detected. Those measurement operations are effected with the band portion being arranged stationarily at the measuring section. The band is then advanced by the length of a portion in order to stationarily position the respective following band portion at the measuring section and to be able to investigate same by applying the multiplicity of current strengths of differing magnitude. By sampling the entire band, band portion by band portion, the individual voltages of the band portions which were respectively obtained by the same electric current I(m) are then added up to obtain a summing voltage. Those summing voltages are ascertained for each electric current of the plurality of different electric currents. The critical current value of the superconducting wire can then be calculated on the basis of a difference voltage.
The method in accordance with EP 860 705 is suitable for investigation of superconductor bands on a laboratory scale but not for production-engineering quality control in which the total production speed of the band including production and quality control of the respective band is at least substantially not to be limited by the quality control procedure. In that respect the production speed of superconductor bands for example by reactive chemical coating with the appropriate buffer and superconductor materials can readily achieve capacities in the region of several kilometers per day for a respective installation. Such a high speed in terms of quality control cannot however be achieved by a method in accordance with EP 860 705 A1.
In addition in production-engineering quality control of superconductor bands it is desirable to locate the defects on the band, that limit the critical transport current, as precisely as possible in order thereby to be able to restore the band, for example by applying a superconducting material in the region of the respective defect or by other suitable measures. That however is not possible with the method of EP 860 705 A1 as here it would be at most possible to ascertain whether there is a relevant defect in a given band portion. That locating operation however would be too inaccurate as then the repair measure would have to involve the entire defective band portion. On the other hand more precise location of the defect is not possible in practice by reducing the length of the respective band portions being investigated, as then, for sufficiently accurate location of the defect, the band portions to be investigated would have to be of such a small length that the band examination time would be unacceptably increased.
U.S. Pat. No. 6,841,988 B1 describes a method and a device for determining the critical current strength of superconducting materials, wherein a superconducting band is transported through a varying external magnetic field. That permits contactless measurement of a magnetic field which is induced in the superconductor band by the external magnetic field. With that contactless measurement procedure the superconductor band is transported continuously through the measuring section. Certain relevant parameters of the superconductor band can admittedly be ascertained by such a method, but quality control in regard to the critical transport current of the band is not possible in that way as the eddy currents generated in the band by the external magnetic field only very indirectly point to defects. Information in regard to the orientation of the defects in relation to the longitudinal direction of the band and thus also a clear reference to a limitation in respect of the critical transport current is not possible in that way.